On Abrupt Impacts of Climate Change: Anticipating Surprises

A report called Abrupt Impacts of Climate Change: Anticipating Surprises produced by the National Research Council for the government warns of tipping points. It also rates as low the possibilities of rapid methane release this century or an abrupt shutdown of the Atlantic Meridional Overturning Circulation.

They dismiss the danger of rapid methane release from permafrost and subsea hydrates this century as a low likelihood. I'm not convinced by their reasoning.

They say that

… the variations in methane through the glacial cycles (1) originated in large part from low-latitude wetlands, and were not dominated by high-latitude sources that could be potentially much larger, and (2) produced a relatively small radiative forcing relative to the temperature changes, serving as a small feedback to climate changes rather than a primary driver. 12

Looking to the future, the available source reservoirs for atmospheric methane release – from both methane hydrates and permafrost – are expected to respond to climate on a time scale slow enough that the climate impact from methane will probably be smaller than that from rising CO2 concentrations. Nevertheless, there is still much to explore. For example a cause for concern is that wildfires have been spreading into some permafrost regions as local climactic conditions promote increasingly dry conditions (Yoshikawa et al., 2002). Little is known about the potential of such burning to thaw and release stored carbon faster than expected. This possible mechanism of rapid, unexpected carbon release merits research to evaluate its efficacy and climactic impacts.

They assume that previous interglacial events, in which warming was spread over thousands of years, are an adequate guide for our current warming that's escalating within decades.

CO2 levels are far higher now than they have been for anytime during the past 800,000 years. Click image to enlarge. Credit: Scripps Institution of Oceanography.

Methane was also proposed as the origin of the Paleocene-Eocene thermal maximum event, 55 million years ago, in which carbon isotopic compositions of CaCO3 shells in deep sea sediments reflect the release of some isotopically light carbon source (like methane or organic carbon), and various temperature proxies indicate warming of the deep ocean and hence the Earth’s surface. But the longevity of the warm period has shown that CO2 was the dominant active greenhouse gas, even if methane was one of the important sources of this CO2 , and the carbon isotope spike shows that if the primary release reservoir were methane, the amount of CO2 that would be produced by this spike was insufficient to explain the extent of warming, unless the climate sensitivity of Earth was much higher than it is today (Pagari et al., 2006).

As I read this, they're arguing that methane could not have been the dominant greenhouse gas because it is short lived while the heating event was long lived. In other words a rapid methane release could not have been the primary driver of the event, because it alone couldn’t explain how long the heat lasted, even if you count the CO2 into which methane decomposes. This ignores the possibility that a methane spike might have initiated other positive feedbacks that continued operating after it had decomposed.

This is like arguing that a match couldn’t have been the primary driver of that house fire, which started when a child held it to the curtains, because the match couldn’t have made the whole house burn down all by itself. ( i.e. curtains don't count)

As for climate sensitivity being higher today, there's reason to think that a rapid belch of methane would dramatically increase methane's radiative forcing, which translates into climate sensitivity.

As far as their call to study what's happening and to try to predict tipping points, *good luck*. Yes it's necessary. But we're in unknown territory. Without quantitative measurements of tipping points in similar historical nonlinear interactions, models will be hard pressed for accurate prediction. My feeling is that politicians will expect scientists to provide exact dates for things no one has ever seen.

Did climate scientists predict the slow down of the polar jet stream, its getting much deeper waves, causing "stuck" storms and even retrograde storm motion, with just .8°C increase? No, we discovered these changes after the fact. Atmosphere and oceans are turbulent systems, resistant to prediction. Emergent phenomena will continue to catch us by surprise.

In my experience, institutions vested in avoiding expensive actions treat deadlines like speed limits. They like to push right up to the edge, and just a bit over if nobody's looking closely. Maximize profit, right now, is an imperative. If they decide that 2°C is a limit, they punt till we 're locked into more than 2°C. This isn't a Hollywood movie. We can't just push right up to a tipping point and stop at the last second, expecting a miraculous save.